Observation of the sodium exosphere from mercury magnetospheric orbiter

The Mercury Sodium Atmosphere Spectral Imager (MSASI) in the BepiColombo mission will address a wealth of fundamental scientific questions pertaining to the Mercury's exosphere. Together, our measurements on the overall scale will provide ample new information on regolith-exosphere-magnetosphere coupling as well as new understanding of the dynamics governing the 'surface-bounded exosphere'. It arises quite clearly from ground-based observations that the regolith of Mercury releases a fraction of its content to the exosphere. Some processes are identified up to now as leading to this ejection, e.g., photon-stimulated desorption. These processes are associated with different energies of ejection and behaviors in different regions of Mercury's surface. Therefore, different types of population are born from the surface, depending on the processes. The MSASI measurements definitely can identify the release processes, how exospheric sodium is born from the regolith. MSASI/BepiColombo is the first and unique opportunity to study the formation, circulation, and maintenance of the surface-bounded exosphere.